Recently, the development of unique sources and detectors in the terahertz (THz) region explored the unique rotational spectral features of compound molecules for toxic gas discrimination and detection. The THz spectral region presents some unique challenges not usually faced in the visible or infrared (IR) regions. Multiple stray unwanted reflections or standing waves may be more severe at THz and the physical optics propagation of the radiation may present a unique challenge to the instrument designer of the light path in the instrument. In particular, THz radiation may not stay collimated over long distances because of its long wavelength. Plane waves may rapidly propagate and translate to spherical waves. Also, beam sizes tend to be larger than their cousins at shorter wavelengths.
For shorter wavelengths, the light source and light path in the so-called White cell are designed to provide multiple bounces between three spherical mirrors, reflectors or the like in its elementary form. One of the three mirrors is often referred to as a field mirror, and the other two mirrors are often referred to as object mirrors. A light source and a detector may be located at symmetric positions about the optical axis of the field mirror and near its surface. The source may be constructed to produce a spherical wave that may be sent to one of the object mirrors. The object mirrors, then, may refocus the beam to the field mirror. The small focused spot (sometimes referred to as an “image”) at a point of reflection on the field mirror may be redirected to the second object mirror as a spherical wave which, in turn, may focus the light back to a new spot on the field mirror. The more spots on the spherical mirror the longer the light path through the cell and the more sensitivity may be provided to a spectrometer in measuring absorption of the gas in the cell. The design then walks the spots over the field mirror until the spot can be placed on the detector. For more information on the White cell, see John U. White, Long Optical Paths of Large Aperture, 32 J. OPT. SOC. AM. 285-288 (1942).
Because of the long wavelength of THz radiation, the spots on the field mirror may be much larger than visible or IR spots. Also, the beam propagation as a function of wavelength may cause severe shifts in focus over a wide band of THz wavelengths. These issues may cause aberrated spots and loss of throughput as the beam divergence may vary significantly with wavelength. The larger spots and packaging constraints may limit the number of light bounces (path length) such a cell can achieve compared to shorter wavelength spectrometers.